It is known to produce aluminide-silicide protective coatings on superalloy turbine blades or turbine vanes to extend the service lives of the turbine blades or turbine vanes.
It is known to produce aluminide-silicide coatings on a superalloy article by depositing a silicon filled organic slurry on the superalloy article and then pack aluminising as described in U.S. Pat. No. 4,310,574. The aluminium carries the silicon from the slurry with it as it diffuses into the superalloy article. Another method of producing aluminide-silicide coatings is by depositing a slurry containing elemental aluminium and silicon metal powders on a superalloy article and then heating to above 760.degree. C. to melt the aluminium and silicon in the slurry, such that they react with the superalloy and diffuse into the superalloy article as described in U.S. Pat. No. 3,248,251. A further method of producing aluminide-silicide coatings is by repeatedly applying the aluminium and silicon containing slurry and heat treating as described in U.S. Pat. No. 5,547,770. Another method of producing aluminide-silicide coatings is by applying a slurry of an eutectic aluminium-silicon or a slurry of elemental aluminium and silicon metal powders on a superalloy article and diffusion heat treating to form a surface layer of increased thickness and reduced silicon content, and a layering layer which comprises alternate interleaved layers of aluminide and silicide phases and a diffusion interface layer as described in published European patent application No. EP0619856A.
It is also known to produce platinum aluminide- silicide coatings on a superalloy article by coating the superalloy article with platinum, then heat treating to diffuse the platinum into the superalloy article and then simultaneously diffusing aluminium and silicon from the molten state into the platinum enriched superalloy article as described in published International patent application No. WO95/23243A. Another method of producing platinum aluminide-silicide coatings on a superalloy article is by coating the superalloy article with platinum, then heat treating to diffuse the platinum into the superalloy article, applying a silicon layer and then aluminising as described in published European patent application No. EP0654542A. It is also possible to diffuse the silicon into the superalloy article with the platinum as described in EP0654542A. A further method of producing platinum aluminide-silicide coatings on a superalloy article is by electrophoretically depositing platinum-silicon powder onto the superalloy article, heat treating to diffuse the platinum and silicon into the superalloy article, then electrophoretically depositing aluminium and chromium powder onto the superalloy article and then heat treating to diffuse the aluminium and chromium into the superalloy article as described in U.S. Pat. No. 5,057,196.
It is also known to produce aluminide coatings on a superalloy article by pack aluminising, out of contact vapour phase aluminising or slurry aluminising. It is also known to produce platinum aluminide coatings by depositing platinum onto the superalloy article and then pack aluminising, out of contact vapour phase aluminising or slurry aluminising.
It has been found that if aluminide, platinum aluminide, aluminide-silicide or platinum aluminide-silicide coatings are produced on high rhenium-containing superalloys, those containing more than 4 wt % rhenium, that topologically close packed phases (TCP phases) are formed within the superalloy substrate. These TCP phases are needle-like rhenium and tungsten rich phases which extend into the substrate. These TCP phases are undesirable because they reduce the useful load bearing area of the superalloy substrate. Also cracking may occur at the interface between the superalloy substrate and the TCP phase leading to decohesion of the aluminide, platinum aluminide, aluminide-silicide or platinum aluminide-silicide coating. Thus the application of these different aluminide coatings onto a high rhenium- containing superalloy article is not practical because these TCP phases increase the stress within the high rhenium-containing superalloy substrate leading to premature failure of the high rhenium-containing superalloy article.
It is further known to deposit MCrAlY or chromium coatings on superalloy turbine blades or turbine vanes to extend the service lives of the turbine blades or turbine vanes. The MCrAlY is generally deposited by plasma spraying or physical vapour deposition, followed by heat treating. The M is at least one of Ni, Co or Fe. The chromium coating is generally deposited by pack or vapour chromising.
It has been found that if a MCrAlY, or chromium, coating is produced on high rhenium-containing superalloys, those containing more than 4 wt % rhenium, that topologically close packed phases (TCP phases) are formed within the superalloy substrate.